Why is everyone complaining?
This game is so simple to understand. You don't need any explanation for this! It's simply fast learning. You notice that there are red enemies coming at you and you shoot automatically red bullets. Your common sense tells you that you shoot the enemies with the same bullet color as the enemies' ship color. But nothing happens so you switch colors and PENG! It dies.
You come to the conclusion that you have to hit it with any other colored bullet.
That's it. You don't need an encyclopedia to explain the gameplay. Especially for a game like this.
Oh, this game is really well made by the way. Thumbs up!
(May contain heavy grammar mistakes or not)